1. Technical Field
The present invention relates to a method of forming a dowel (the dowel being equal to half blanking) section functioning as a tongue guide in a base member of a buckle for attaching a seat belt provided on a seat of an automobile or a vehicle for transportation.
2. Description of the Related Art
A seat of an automobile or various types of transportation vehicles is equipped with a seat belt for protecting the occupant of the seat from the impact at the time of a collision. In a seat belt, a tongue and a buckle are mounted to a seat belt web. By inserting the tongue into the buckle and locking it therein, the seat belt web is wound around the body of the occupant to constrain the same.
In the buckle, a latch member is rotatably mounted to a base member receiving the tongue, and the latch member latching the tongue is urged in the latching direction by a spring (see, for example, JP 2001-80462 A).
Abase member 11 is bent into a U-shaped member in which both side walls are opposed to each other, and inwardly protruding guide portions 11a and 11b are respectively provided on the side walls (FIG. 1). In the engagement state at the time of latching (FIG. 2), the guide portions 11a and 11b exhibit a function of guiding the inserted tongue 12. They also function as a guide and a load receiver when a latch member 13 is elongated at the time of generation of an impact or the like.
Conventional guide portions are formed as inwardly protruding dowel sections by performing press working on a part of the side walls of the base member. Recently, however, the following features are required of a buckle base member: a dowel height of approximately 90% of the plate thickness as measured from the inner surface of the base side wall; a dowel section side surface shear cross-section length L of 25% or more of the plate thickness; and a shear cross-section starting height H of 70% or more of the plate thickness. Thus, the recent requirements for the dowel section performance are rather severe. It should be noted that the shear cross-section length L refers to the length in the plate thickness direction of the shear surface appearing at the cut end surface, and the shear cross-section starting height H refers to the length in the plate thickness direction as measured from the dowel section root to the shear cross-section starting position (FIG. 4).
When a dowel section, of which there are severe requirements in terms of configuration, dimension, surface property, and the like, is formed by a conventional press working method, a crack is generated and allowed to grow from the vicinity of the punch and the die cutter at the time of working, and eventually becomes a crack extending through the plate thickness to promote exposure of a rupture surface. Exposure of the rupture surface reduces the effective contact area at the component contact position, deteriorates the smoothness of the sliding surface in the slide portion, and hinders smooth sliding motion of the tongue and the dowel section side surface.